1. Field of the Invention
The invention relates to a battery that includes a safety valve that is opened when pressure in the battery increases, and a method of assembling the battery.
2. Description of the Related Art
When a battery is used, gas is generated in the battery due to an electrode reaction. The pressure in the battery increases due to the generated gas. The pressure in the battery increases particularly due to the over-charge or over-discharge of the battery. In general, a sealed battery is provided with a safety valve that opens a part of a casing to prevent an excessive increase in the pressure in the battery when the pressure in the battery increases.
Japanese Patent Application Publication No. JP-A-10-106524 (hereinafter, referred to as “Publication No. 10-106524”) describes a device in which a safety valve is formed in a casing. In the device, a thin area is formed in a cover made of a metallic plate. The thin area is the thinnest area in the cover. The thin area functions as the safety valve. When the pressure in the battery increases, a part of the thin area is broken to prevent an excessive increase in the pressure in the battery. Japanese Patent Application Publication No. JP-A-9-293490 (hereinafter, referred to as “Publication No. 9-293490”) describes a battery and a method of assembling the battery, in which the peripheral edge portion of a safety valve is welded to the peripheral edge portion around a vent hole of a casing.
When the safety valve is formed in a part of the casing made of metal as described in Publication No. 10-106524, or when the safety valve is welded to the casing made of metal as described in Publication No. 9-293490, impacts applied to the casing, or vibrations caused in the casing are directly transmitted to the safety valve. The safety valve is more fragile than the body of the casing. If the impacts applied to the casing are directly transmitted to the safety valve, the safety valve may be broken. If the vibrations caused in the casing are directly transmitted to the safety valve, the safety valve may deteriorate. If the safety valve deteriorates, the safety valve may be opened even when the pressure in the battery is below a predetermined value.